Methods of preparing an absorption layer of a solar cell using a material such as Cu(In,Ga)Se2 (abbreviated as CIGS-based absorption layer) as an absorption layer may be broadly divided into two methods, i.e., one method involving vacuum deposition, and the other method involving applying a precursor material under non-vacuum conditions, followed by high-temperature thermal processing. Of these methods, the former vacuum deposition method is advantageously capable of preparing a high-efficiency absorption layer, but suffers from shortcomings such as low uniformity upon the preparation of an absorption layer having a large area and a need for use of expensive equipment. Whereas, the latter method including high-temperature thermal processing following application of the precursor material is capable of uniformly preparing a large-area absorption layer, but disadvantageously results in a poor efficiency of the absorption layer.
As a method that is most feasible to be put to practical use for the industrial scale production of an absorption layer among the above-mentioned methods for preparing an absorption layer utilizing a precursor material, mention may be made of a method of preparing an absorption layer by coating a paste of a metal oxide mixture on a base material and subjecting the coating to thermal processing. This method provides an advantage in that a uniform absorption layer can be prepared at low production costs, but the metal oxides used as the precursor are very highly stable in terms of chemical and thermal properties thereof and therefore it is difficult to achieve large-sized crystals in the final absorption layer, consequently leading to a decreased efficiency of the absorption layer.